The wrestling match between Heracles and Achelous.
They say that Heracles fought Achelous in single combat. But here is how it was: the Achelous flowed between the Aetolians and the Curetes and would cut off great tracts of land, sometimes favouring one tribe and sometimes the other. As a result, a great quarrel arose. Heracles, coming to the Aetolians as an ally, defeated the Curetes and, having confined the river in a single channel and one outlet, he strengthened the land to the advantage of the Aetolians and took away Deianeira, daughter of Oeneus.
De Incredibilibus (excerpta Vaticana). Anonymous. Oxford. 2014.